How to tell if a reaction is endothermic or exothermic?

Which sentence about particles in matter is true? a. atoms are present in solids and liquids but not in gases. b. the particles of matter are in constant motion. c. the same kinds of atoms are found in different elements. d. when a solid changes to a liquid, the sizes of the particles change.

Explain in detail of the four major scientific developments that spurred the formulation of the plate tectonics theory

How many valence electrons are in atom of radon?